Systems and methods for detecting denial of service attacks

ABSTRACT

A method for detecting malicious packets includes comparing a sequence number of an input packet with sequence numbers of one or more previously input packets and detecting a malicious packet when the sequence number of the input packet matches at least one of the sequence numbers of the previously input packets.

REFERENCE TO RELATED APPLICATION

This application is based on and claims the benefit of ProvisionalApplication Ser. No. 60/572,683 filed May 20, 2004, the entire contentsof which are herein incorporated by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to denial of service attacks and, morespecifically, to systems and methods for detecting denial of serviceattacks.

2. Description of the Related Art

Computer systems and computer networks are commonly used by corporationsand institutions to store and manage sensitive information.Additionally, computer systems and networks are often used to providethe constituents of corporations and institutions with around-the-clockaccess to information and services through the use of websites andrelated web-based services.

While the use of such systems and networks has many advantages, thesesystems and networks present a risk to corporations and institutionsthat their systems and networks can be exploited or vandalized bymalicious attack. Malicious attacks can be attempts by individuals towillfully and/or maliciously exploit and/or damage the public or privatesystems and networks of others. Malicious attacks may be launched byindividuals wishing to do harm or by unscrupulous competitors desiring acompetitive advantage. Malicious attacks can potentially compromisesensitive data, damage software and/or hardware, tie up valuable networkresources and disrupt the availability of websites and related web-basedservices.

One particularly popular form of malicious attack is the denial ofservice attack. The denial of service attack is a broad term used todescribe malicious attacks that attempt to disrupt the service ofwebsites and related web-based services generally by bogging downtargeted servers with a barrage of bogus requests for information. Therecan be many forms of denial of service attacks. Examples of denial ofservice attacks include buffer overflow attacks, the SYN Attack, theTeardrop attack and the Smurf attack.

The buffer overflow attack is among the most common forms of denial ofservice attack. This attack attempts to send a server more networktraffic than it has been designed to accept. Network traffic can be datathat is sent to a server by a system or user wishing to establish acommunication connection. Network traffic can be data that istransmitted during the course of communication. Servers, such as webservers and email servers, often use a data buffer to temporarily storetraffic that is received while the server is busy processing othertraffic. Data buffers have a limited capacity that can be selected basedon the volume of traffic that the server has been designed toaccommodate. By sending a barrage of network traffic to a server, theserver's data buffer can overflow. When a data buffer overflows, thedata buffer may become incapable of receiving additional traffic becausethere may not be additional free space to accommodate the traffic.Traffic that is sent to a server with an overflowing data buffer may notbe accepted and the server may therefore not be in a usable state.

Buffer overflow attacks may additionally exploit weaknesses in thesystem targeted to increase the effectiveness of the attack. Suchattacks may simply send a value that is larger than the target server isprepared to accommodate. Because servers are generally designed tofunction correctly even when presented with values that are too large,creating a buffer overflow of this nature involves exploiting a specificweakness in the server being targeted. For example, buffer overflowattacks may send email messages to the target server that containattachments with 256-character file names thereby exploiting a weaknessthat has been identified in certain email servers and more easilycausing a buffer overflow. Other email servers can be vulnerable toemails that have been addressed to an email address exceeding 256characters.

The use of such denial of service attacks is wide spread and may begrowing and the threat posed by such attacks is large. An efficient andeffective way of protecting computer systems and networks from thisgrowing threat is highly desirable.

SUMMARY

A method for detecting malicious packets includes comparing a sequencenumber of an input packet with sequence numbers of one or morepreviously input packets and detecting a malicious packet when saidsequence number of said input packet matches at least one of saidsequence numbers of said one or more previously input packets.

A method for detecting malicious packets includes comparing anacknowledgment number of an input packet with acknowledgment numbers ofone or more previously input packets and detecting a malicious packetwhen said acknowledgment number of said input packet matches at leastone of said acknowledgment numbers of said previously input packets.

A method for detecting malicious packets includes comparing a sequencenumber of an input packet with sequence numbers of one or morepreviously input packets, comparing an acknowledgment number of saidinput packet with acknowledgment numbers of said one or more previouslyinput packets and detecting a malicious packet when said sequence numberof said input packet matches at least one of said sequence numbers ofsaid previously input packets and/or said acknowledgement number of saidinput packet matches at least one of said acknowledgement numbers ofsaid previously input packets.

A system for detecting malicious packets includes a comparing unit forcomparing a sequence number of an input packet with sequence numbers ofone or more previously input packets and a detecting unit for detectinga malicious packet when said sequence number of said input packetmatches at least one of said sequence numbers of said previously inputpackets.

A system for detecting malicious packets includes a comparing unit forcomparing an acknowledgment number of an input packet withacknowledgment numbers of one or more previously input packets and adetecting unit for detecting a malicious packet when said acknowledgmentnumber of said input packet matches at least one of said acknowledgmentnumbers of said previously input packets.

A system for detecting malicious packets includes a sequencenumber-comparing unit for comparing a sequence number of an input packetwith sequence numbers of one or more previously input packets, anacknowledgment number-comparing unit for comparing an acknowledgmentnumber of said input packet with acknowledgment numbers of said one ormore previously input packets and a detecting unit for detecting amalicious packet when either said sequence number of said input packetmatches at least one of said sequence numbers of said previously inputpackets and/or said acknowledgement number of said input packet matchesat least one of said acknowledgement numbers of said previously inputpackets.

A computer system includes a processor and a computer recording mediumincluding computer executable code executable by the processor fordetecting malicious packets. The computer executable code includes codefor comparing a sequence number of an input packet with sequence numbersof one or more previously input packets and code for detecting amalicious packet when said sequence number of said input packet matchesat least one of said sequence numbers of said previously input packets.

A computer system includes a processor and a computer recording mediumincluding computer executable code executable by the processor fordetecting malicious packets. The computer executable code includes codefor comparing an acknowledgment number of an input packet withacknowledgment numbers of one or more previously input packets and codefor detecting a malicious packet when said acknowledgment number of saidinput packet matches at least one of said acknowledgment numbers of saidpreviously input packets.

A computer system includes a processor and a computer recording mediumincluding computer executable code executable by the processor fordetecting malicious packets. The computer executable code includes codefor comparing a sequence number of an input packet with sequence numbersof one or more previously input packets, code for comparing anacknowledgment number of said input packet with acknowledgment numbersof said one or more previously input packets and code for detecting amalicious packet when said sequence number of said input packet matchesat least one of said sequence numbers of said previously input packetsand/or said acknowledgement number of said input packet matches at leastone of said acknowledgement numbers of said previously input packets.

A computer recording medium including computer executable code fordetecting malicious packets, includes code for comparing a sequencenumber of an input packet with sequence numbers of one or morepreviously input packets and code for detecting a malicious packet whensaid sequence number of said input packet matches at least one of saidsequence numbers of said one or more previously input packets.

A computer recording medium including computer executable code fordetecting malicious packets, includes code for comparing anacknowledgment number of an input packet with acknowledgment numbers ofone or more previously input packets and code for detecting a maliciouspacket when said acknowledgment number of said input packet matches atleast one of said acknowledgment numbers of said previously inputpackets.

A computer recording medium including computer executable code fordetecting malicious packets, includes code for comparing a sequencenumber of an input packet with sequence numbers of one or morepreviously input packets, code for comparing an acknowledgment number ofsaid input packet with acknowledgment numbers of said one or morepreviously input packets and code for detecting a malicious packet whensaid sequence number of said input packet matches at least one of saidsequence numbers of said previously input packets and/or saidacknowledgement number of said input packet matches at least one of saidacknowledgement numbers of said previously input packets.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present disclosure and many of theattendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

FIG. 1 shows the structure of a TCP header;

FIG. 2 shows how a device can be protected from malicious packets thatform a denial of service attack according to embodiments of the presentdisclosure;

FIG. 3 shows a method by which packets can be analyzed and blocked whenthe packets are determined to be malicious according to an embodiment ofthe present disclosure;

FIGS. 4A-4C show examples of a sequence number database according to anembodiment of the present disclosure;

FIG. 5 shows a method by which packets can be analyzed and blocked whenthe packets are determined to be malicious according to anotherembodiment of the present disclosure; and

FIG. 6 shows an example of a computer system capable of implementing themethod and apparatus according to embodiments of the present disclosure.

DETAILED DESCRIPTION

In describing the preferred embodiments of the present disclosureillustrated in the drawings, specific terminology is employed for sakeof clarity. However, the present disclosure is not intended to belimited to the specific terminology so selected, and it is to beunderstood that each specific element includes all technical equivalentswhich operate in a similar manner.

While denial of service attacks can theoretically originate from withinthe local network where the targeted server is located, it is mostcommon for denial of service attacks to be launched from beyond thelocal network, for example, from over the internet.

Computer networks often use a set of communications protocols to definehow information is communicated within the network. Many networks, forexample the internet, use the Transmission Control Protocol/InternetProtocol (TCP/IP) set of communications protocols. The most commonlyused protocols within this set are the Transmission Control Protocol(TCP) and the Internet Protocol (IP). The TCP/IP protocol setcommunicates data along various layers of the computer network bybreaking up data into discrete units of data called packets andcommunicating each unit of data individually in accordance with theprotocols that form TCP/IP. The form of the packet that should be useddepends on the protocols responsible for the particular layer ofcommunication. Packets of data communicated by one protocol layer may becommunicated via packets of data communicated by another protocol layerupon which the first protocol layer is built. For example, TCP/IPpackets may be sent by application layer protocols. These protocols aregenerally concerned with how the user perceives the network.

Within the TCP/IP protocol set, the IP protocol is primarily responsiblefor the routing of data. Packets communicated by the IP protocol, maycontain an IP header to facilitate packet routing. This header may be aset of information with a well defined structure that contains, forexample, the address the packet is intended to be delivered to(destination address) and the address where the packet has originatedfrom (source address).

The TCP protocol generally operates on top of the IP protocol and isprimarily responsible for maintaining a reliable communication usingmeasures to verify that packets have been correctly received. Innetworks employing the TCP/IP protocol set (TCP/IP networks), the IPheader is generally followed by the TCP header to facilitate reliablecommunication. The TCP header may be a set of information with a welldefined structure that contains information useful to the TCP protocolin maintaining reliable communication.

FIG. 1 shows the structure of the TCP header 100. A TCP header can befound in the beginning of each packet generated for communication by theTCP protocol (TCP packet). TCP packets are often called TCP segments andare an example of the category of packets referred to as datagrams. TheTCP header is generally comprised of a series of 32-bit numbers oftenreferred to as “words”. As illustrated in FIG. 1, every row represents asingle 32-bit word. Where a row is comprised of more than one section,as is the case with the row comprising the source port 101 and thedestination port 102, each section represents a word of less than32-bits and all of the sections of a row together add to 32-bits.

The source port number 101 is generally a 16-bit word indicating theport of communication used by the system that generated the packet. Thedestination port number 102 is generally a 16-bit word that indicatesthe port of communication used by the system that the packet isaddressed to.

The sequence number 103 is generally a 32-bit word used to keep track ofthe correct order of the packets. The sequence number may begin as aninitial sequence number (ISN). The ISN is generally a randomly generatednumber that can be used as the sequence number on the first TCP packetsent for a communication. Each subsequent TCP packet sent as part of thesame communication uses a sequence number equal to the ISN plus thenumber of bytes that have so far been sent as part of the samecommunication.

The acknowledgement number 104 is generally a 32-bit number indicatingthe value of the next sequence number 103 the sender of the TCP packetis expecting to receive. The data offset 105 is generally a 4-bit numberindicating the total size of the TCP header by indicating how many32-bit words the TCP header is comprised of. The reserved space 106 isgenerally a 6-bit value reserved for future use. The reserve space 106should be set at zero. The control bits 107 are generally comprised of 61-bit values (switches) where URG is generally the urgent pointer fieldsignificant switch, ACK is generally the acknowledgement fieldsignificant switch, PSH is generally the push function switch, RST isgenerally the reset the connection switch, SYN is generally thesynchronize sequence numbers and FIN is generally the no more data fromsender switch. The window 108 is generally a 16-bit number indicatingthe amount of data the sender of the TCP packet should be willing toaccept. The checksum 109 is generally a 16-bit number that is generallyused to ensure that the TCP header has been correctly received. Theurgent pointer 110 is generally a 16-bit number that is generally usedto point to urgent data. This field should only be used in TCP packetswhere the URG switch of the control bits 107 should be set to 1 (on).The options 111 may be an 8-bit word, a 16-bit word, a 24-bit word or a32-bit word. Where the options 111 is less than a 32-bit word, padding112 fills the remaining space on the 32-bit word with zeroes. The option111 allows for additional TCP header features. The data field 113 isgenerally not part of the TCP header but instead shows where the dataassociated with the TCP packet begins. TCP packet data is generallycomprised of a series of 32-bit words.

Embodiments of the present disclosure automatically analyze and blockmalicious packets of data that are determined to be part of a maliciousattack, for example a denial of service attack so that the maliciouspackets can be prevented from reaching the device that has been targetedfor attack (the targeted device).

FIG. 2 shows how the targeted device can be protected from maliciouspackets that form a denial of service attack according to embodiments ofthe present disclosure. The protected device 204 may be, for example, aserver hosting a website. Because the denial of service attack mostlikely arrives from the internet 201, a firewall 203 can be placedbetween the internet 201 and the protected device 204. A firewall 203can be a device that can be programmed to reject (block) particularpackets of data. The firewall 203 can be software running on a device,for example a computer, or it can be a dedicated hardware device. Thefirewall 203 can be programmed to block packets that have beendetermined to be part of a denial of service attack. The determinationas to what packets are part of a denial of service attack can be made bya comparer 202 which analyzes packets according to embodiments of thepresent disclosure. After the comparer 202 has determined what packetsare part of the attack, information concerning how to identify thesemalicious packets can be sent to the firewall 203 so that the maliciouspackets can be blocked from reaching the protected device 204. Comparer202 can be a part of the firewall and as such can be software running ona device, for example a computer, or can be a dedicated hardware device.

FIG. 3 illustrates a method by which packets can be analyzed by thecomparer 202 and blocked by the firewall 203 when the packets aredetermined to be malicious according to an embodiment of the presentdisclosure. According to an embodiment of the present disclosure, adetermination can be made as to whether the data is part of a denial ofservice attack by examining the sequence number (Step S301) of the dataand/or by examining the acknowledgement number (Step S311) of the data.This embodiment carries out two sets of method steps in parallel, oneset of method steps relating to the sequence numbers of packets (StepsS301-S308) and the other set of method steps relating to acknowledgmentnumbers of packets (Steps S311 -S318). According to an embodiment, apacket may be blocked based on the sequence number (Step S305), based onthe acknowledgement number (Step S315) or based on both the sequencenumber and the acknowledgement number. Other embodiments of the presentdisclosure may use only the method steps relating to the sequencenumbers (Steps S301-S308) to block packets. Other embodiments of thepresent disclosure may use only the method steps relating to theacknowledgement numbers (Step S311-S318) to block packets.

According to this embodiment of the present disclosure, data that is tobe received by the protected device 204 via the internet 201 can beexamined to ascertain the sequence number 103 (Step S301) of the TCPheader 100 for each incoming TCP packet and/or to ascertain theacknowledgement number 104 (Step S311) of the TCP header 100 for eachincoming TCP packet.

The sequence number 103 can then be compared against entries within asequence number database 306 (Step S302). According to an embodiment ofthe present disclosure, the sequence number database stores the sequencenumbers 103 of the most recent packets that have been previouslyexamined. Comparing (Step S302) can be facilitated by the use ofdatabase search techniques, for example, a hash table. A hash table isan index that allows for quicker searching of the entries within thesequence number database 306 so that only the index needs be searchedand not the database entries themselves.

The sequence number database 306 may store a fixed number of the mostrecent sequence numbers 103 detected. After the sequence number database306 has been filled to capacity, room can be created for the nextsequence number 103 by removing the oldest sequence number from thedatabase 306. Therefore, the sequence number database can be afirst-in-first-out (FIFO) array. For example, the oldest sequence numberfrom the sequence database 306 can be replaced with the next sequencenumber 103 and the hash index can be changed to reflect the change ofvalues.

An example of a sequence number database 306 according to an embodimentof the present disclosure is illustrated in FIG. 4A-FIG. 4C. FIG. 4Ashows an array of 6 32-bit words (shown in hexadecimal format forsimplicity) forming an example sequence number database 401. Althoughthis example only contains 6 entries, embodiments of the presentdisclosure may contain any number of entries. The 6 32-bit wordsrepresent the sequence numbers of the last 6 packets to be analyzed.FIG. 4B shows the next sequence number 403 to be added to the sequencenumber database 402. As shown in FIG. 4C, the next sequence number 403can be added to the top of the sequence number database 404 therebypushing out the oldest sequence number entry 405.

As described above, the sequence number 103 can either be a randomlygenerated 32-bit word (ISN) (when the TCP packet is the first TCP packetof a communication) or a number formed by adding the number of bytestransmitted to the ISN. The result is that the same 32-bit sequencenumber is likely to reoccur less frequently than once every four billionnew communications. Therefore, even if the sequence number database 306contains the last tens of thousands of sequence numbers 103 detected,the examined sequence number 103 should not match a sequence numberstored in the sequence number database 306.

Many programs used to launch denial of service attacks are written tocreate the TCP header manually instead of relying on the TCP/IPprotocols to generate random ISNs. Therefore, packets produced by denialof service attacks commonly contain identical sequence numbers 103.Therefore when the examined sequence number 103 is compared to thesequence numbers 103 in the sequence number database 306 (Step S302),sequence numbers 103 of the packets that are part of a denial of serviceattack will likely match a sequence number stored in the sequence numberdatabase 306.

Where the sequence number 103 of the packet does not match a sequencenumber 103 in the sequence number database 306 (No Step 303), thesequence number 103 of the packet can be added to the sequence numberdatabase 306 for the benefit of subsequent iterations of the instantmethod (Step S307). Such packets will be allowed to continue through thefirewall 203 and to the protected device 264 (Step S308) provided thepacket is not blocked as a result of the acknowledgment number methodsteps (Step S315) as described below.

Where the sequence number 103 of the packet matches a sequence number103 in the sequence number database 306 (Yes Step 303), the sequencenumber 103 of the packet may be added to the sequence number database306 (Step S304) so that protection from the denial of service attack cancontinue for as long as the attack is in progress. Such packets will beblocked by the firewall 203 and will therefore be prevented fromreaching the protected device 204 (Step S305) regardless of whether thepacket is blocked as a result of the acknowledgement number method steps(Step S315) as described below.

As stated above, according to the embodiment of the present disclosureshown in FIG. 3, the method steps relating to the sequence numbers ofpackets (Steps S301-S308) and the method steps relating to theacknowledgement numbers of packets (Steps S311-S318) can be carried outin parallel. Therefore, as the method steps relating to the sequencenumbers of packets (Steps S301-S308) as described above are carried out,the acknowledgement number 104 can then be compared against each entrywithin an acknowledgement number database 316 (Step S312). Theacknowledgement number database 316 stores the acknowledgement numbers104 of the most recent packets that have been examined according to thepresent disclosure. Comparing (Step S312) can be facilitated by the useof database search techniques, for example, a hash table.

Because the acknowledgement number database 316 stores a fixed number ofthe most recent acknowledgement numbers 104 detected, after theacknowledgement number database 316 has been filled to capacity, roomcan be created for the next acknowledgement number 104 by removing theoldest acknowledgement number from the database 316. Therefore, theacknowledgement number database can be a first-in-first-out (FIFO)array. For example, the oldest acknowledgement number from theacknowledgement database 316 can be replaced with the nextacknowledgement number 104 and the hash index can be changed to reflectthe change of values.

As described above, the sequence number 103 can either be a randomlygenerated ISN or a sequential derivative of the ISN. As described above,the acknowledgement number 104 is also generally a sequential derivativeof the ISN. Therefore, there is a similarly infinitesimal probabilitythat two packets derived according to the TCP/IP protocol set would haveidentical acknowledgement numbers 104. Therefore, even if theacknowledgement number database 316 contains the last tens of thousandsof acknowledgement numbers 104 detected, the examined acknowledgementnumber 104 should not match an acknowledgement number stored in theacknowledgement number database 316.

For the reasons stated above, packets produced by denial of serviceattacks commonly contain identical acknowledgement numbers 104.Therefore when the examined acknowledgement number 104 can be comparedto the acknowledgement numbers 104 in the acknowledgement numberdatabase 316 (Step S312), acknowledgement numbers 104 of the packetsthat are part of a denial of service attack will likely match anacknowledgement number stored in the acknowledgement number database316.

Where the acknowledgement number 104 of the packet does not match anacknowledgement number 104 in the acknowledgement number database 316(No Step 313), the acknowledgement number 104 of the packet can be addedto the acknowledgement number database 316 for the benefit of subsequentiterations of the instant method (Step S317). Such packets will beallowed to continue through the firewall 203 and onto the protecteddevice 204 (Step S318) provided that the same packet was allowed withrespect to its sequence number (Step S308) as described above.

Where the acknowledgement number 104 of the packet matches anacknowledgement number 104 in the acknowledgement number database 316(Yes Step 313), the acknowledgement number 104 of the packet may beadded to the acknowledgement number database 316 (Step S314) so thatprotection from the denial of service attack can continue for as long asthe attack is in progress. Such packets will be blocked by the firewall203 and will therefore be prevented from reaching the protected device204 (Step S315) regardless of whether the packet is blocked as a resultof the sequence number method steps (Step S305) as described above.

It should be noted that the sequence number of a packet may be the sameas a previous sequence number if the client did not send any data in theprevious packet. Similarly, the acknowledgement number may be the sameas a previous acknowledgement number if the server did not send any datain its previous packet. The following embodiment described by referenceto FIG. 5, is similar to the embodiment described in FIG. 3. Likenumbers represent the same or similar steps to those described abovewith reference to FIG. 3. Accordingly, not all steps in FIG. 5 will bedescribed below. According to this embodiment of the present disclosure,after the sequence number is examined (Step S301), a determination ismade whether there is data in the packet (Step S500). If there is datain the packet (Yes, Step S500), the sequence number is compared to thesequence numbers of the sequence number database 306 (Step S302) and theprocess continues as described above with reference to FIG. 3. If thereis no data in the packet (No, Step S500), the packet is allowed to pass(Step S502). However, the sequence number is not added to the sequencenumber database. In a similar fashion, after the acknowledgement numberis examined (Step S311), a determination is made whether there is datain the packet (Step S504). If there is data in the packet (Yes, StepS504), the acknowledgement number is compared to the acknowledgementnumbers of the acknowledgement number database 306 (Step S312) and theprocess continues as described above with reference to FIG. 3. If thereis no data in the packet (No, Step S504), the packet is allowed to pass(Step S506). However, the acknowledgement number is not added to theacknowledgement number database.

According to another embodiment, where the client did not send data tothe server and only the SYN flag is set, the sequence number of thepacket can be added to the sequence number database and the packetprocessed like the other packets. That is, the packet is allowed to passonly if the sequence number was not already in the database before thepacket arrived.

FIG. 6 shows an example of a computer system which may implement themethod and system of the present disclosure. The system and method ofthe present disclosure may be implemented in the form of a softwareapplication running on a computer system, for example, a mainframe,personal computer (PC), handheld computer, server, etc. The softwareapplication may be stored on a recording media locally accessible by thecomputer system and accessible via a hard wired or wireless connectionto a network, for example, a local area network, or the Internet.

The computer system referred to generally as system 600 may include, forexample, a central processing unit (CPU) 602, random access memory (RAM)604, a printer interface 606, a display unit 608, a local area network(LAN) data transmission controller 610, a LAN interface 612, a networkcontroller 614, an internal buss 616, and one or more input devices 618,for example, a keyboard, mouse etc. As shown, the system 600 may beconnected to a data storage device, for example, a hard disk, 620 via alink 622.

1. A method for detecting malicious packets, the method comprising:comparing a sequence number of an input packet with sequence numbers ofone or more previously input packets; and detecting a malicious packetwhen said sequence number of said input packet matches at least one ofsaid sequence numbers of said one or more previously input packets. 2.The method according to claim 1, wherein a database of sequence numbersof previously input packets is used in comparing said sequence number ofsaid input packet with said sequence numbers of said one or morepreviously input packets and further comprising: determining whether theinput packet has data; and adding said sequence number of said inputpacket to said database of sequence numbers when it is determined thatthere is data in the packet.
 3. The method according to claim 2, whereinsaid database of sequence numbers is a database of a fixed size suchthat when said database of sequence numbers is filled to capacity,adding said sequence number of said input packet to said database ofsequence numbers further comprises removing an oldest sequence numberfrom said database of sequence numbers prior to adding said sequencenumber of said input packet to said database of sequence numbers.
 4. Themethod according to claim 2, further comprising: using a hash table insaid comparing said sequence number of said input packet with saidsequence numbers of said one or more previously input packets.
 5. Themethod according to claim 1, further comprising: blocking said detectedmalicious packet.
 6. The method according to claim 5, wherein blockingsaid detected malicious packet comprises instructing a firewall toreject said detected malicious packets.
 7. A method for detectingmalicious packets, the method comprising: comparing an acknowledgmentnumber of an input packet with acknowledgment numbers of one or morepreviously input packets; and detecting a malicious packet when saidacknowledgment number of said input packet matches at least one of saidacknowledgment numbers of said previously input packets.
 8. The methodaccording to claim 7, wherein a database of acknowledgment numbers ofpreviously input packets is used in comparing said acknowledgment numberof said input packet with said acknowledgment numbers of said one ormore previously input packets and said method for detecting maliciouspackets additionally comprises: determining whether the input packet hasdata; and adding said acknowledgment number of said input packet to saiddatabase of acknowledgment numbers when it is determined that there isdata in the packet.
 9. The method according to claim 8, wherein saiddatabase of acknowledgment numbers is a database of a fixed size suchthat when said database of acknowledgment numbers is filled to capacity,adding said acknowledgment number of said input packet to said databaseof acknowledgment numbers additionally comprises removing an oldestacknowledgment number from said database of acknowledgement numbersprior to adding said acknowledgment number of said input packet to saiddatabase of acknowledgment numbers.
 10. The method according to claim 8,wherein further comprising: using a hash table when comparing saidacknowledgment number of said input packet with said acknowledgmentnumbers of said one or more previously input packets.
 11. The methodaccording to claim 7, further comprising: blocking said detectedmalicious packet.
 12. The method according to claim 11, wherein saidblocking of said detected malicious packet comprises: instructing afirewall to reject said detected malicious packets.
 13. A method fordetecting malicious packets, the method comprising: comparing a sequencenumber of an input packet with sequence numbers of one or morepreviously input packets; comparing an acknowledgment number of saidinput packet with acknowledgment numbers of said one or more previouslyinput packets; and detecting a malicious packet when said sequencenumber of said input packet matches at least one of said sequencenumbers of said previously input packets and/or said acknowledgementnumber of said input packet matches at least one of said acknowledgementnumbers of said previously input packets.
 14. The method according toclaim 13, wherein a database of sequence numbers of previously inputpackets is used in comparing said sequence number of said input packetwith said sequence numbers of said one or more previously input packets;a database of acknowledgment numbers of previously input packets is usedin comparing said acknowledgment number of said input packet with saidacknowledgment numbers of said one or more previously input packets;determining whether there is data in said input packet; and said methodfor detecting malicious packets further comprises: adding said sequencenumber of said input packet to said database of sequence numbers when itis determined that there is data in said input packet; and adding saidacknowledgment number of said input packet to said database ofacknowledgment numbers when it is determined that there is data in saidinput packet.
 15. The method according to claim 14, wherein saiddatabase of sequence numbers is a database of a fixed size such thatwhen said database of sequence numbers is filled to capacity, addingsaid sequence number of said input packet to said database of sequencenumbers additionally comprises removing an oldest sequence number fromsaid database of sequence numbers prior to adding said sequence numberof said input packet to said database of sequence numbers; and saiddatabase of acknowledgment numbers is a database of a fixed size suchthat when said database of acknowledgment numbers is filled to capacity,adding said acknowledgment number of said input packet to said databaseof acknowledgment numbers additionally comprises removing an oldestacknowledgment number from said database of acknowledgement numbersprior to adding said acknowledgment number of said input packet to saiddatabase of acknowledgment numbers.
 16. The method according to claim14, further comprising: using a hash table when comparing said sequencenumber of said input packet with said sequence numbers of said one ormore previously input packets; and using a hash table when comparingsaid acknowledgment number of said input packet with said acknowledgmentnumbers of said one or more previously input packets.
 17. The methodaccording to claim 13, further comprising: blocking said detectedmalicious packet.
 18. The method according to claim 17, wherein blockingdetected malicious packet comprises: instructing a firewall to rejectsaid detected malicious packets.
 19. A system for detecting maliciouspackets, the system comprising: a comparing unit for comparing asequence number of an input packet with sequence numbers of one or morepreviously input packets; and a detecting unit for detecting a maliciouspacket when said sequence number of said input packet matches at leastone of said sequence numbers of said previously input packets.
 20. Thesystem according to claim 19, wherein a database of sequence numbers ofpreviously input packets is used in comparing said sequence number ofsaid input packet with said sequence numbers of said one or morepreviously input packets and further comprising: a determining unit fordetermining whether there is data in said input packet; and an addingunit for adding said sequence number of said input packet to saiddatabase of sequence numbers when it is determined that there is data insaid input packet.
 21. The system according to claim 20, wherein saiddatabase of sequence numbers is a database of a fixed size such thatwhen said database of sequence numbers is filled to capacity, addingsaid sequence number of said input packet to said database of sequencenumbers further comprises removing an oldest sequence number from saiddatabase of sequence numbers prior to adding said sequence number ofsaid input packet to said database of sequence numbers.
 22. The systemaccording to claim 20, wherein said comparing unit uses a hash tablewhen comparing said sequence number of said input packet with saidsequence numbers of said one or more previously input packets.
 23. Thesystem according to claim 19, further comprising: a blocking unit forblocking said detected malicious packet.
 24. The system according toclaim 23, wherein said blocking unit comprises a firewall which isinstructed to reject said detected malicious packets.
 25. A system fordetecting malicious packets, the system comprising: a comparing unit forcomparing an acknowledgment number of an input packet withacknowledgment numbers of one or more previously input packets; and adetecting unit for detecting a malicious packet when said acknowledgmentnumber of said input packet matches at least one of said acknowledgmentnumbers of said previously input packets.
 26. The system according toclaim 25, wherein a database of acknowledgment numbers of previouslyinput packets is used in comparing said acknowledgment number of saidinput packet with said acknowledgment numbers of said one or morepreviously input packets and further comprising: a determining unit fordetermining whether there is data in said input packet; and an addingunit for adding said acknowledgment number of said input packet to saiddatabase of acknowledgment numbers when it is determined that there isdata in said input packet.
 27. The system according to claim 26, whereinsaid database of acknowledgment numbers is a database of a fixed sizesuch that when said database of acknowledgment numbers is filled tocapacity, adding said acknowledgment number of said input packet to saiddatabase of acknowledgment numbers additionally comprises removing anoldest acknowledgment number from said database of acknowledgementnumbers prior to adding said, acknowledgment number of said input packetto said database of acknowledgment numbers.
 28. The system according toclaim 26, wherein said comparing unit uses a hash table when comparingsaid acknowledgment number of said input packet with said acknowledgmentnumbers of said one or more previously input packets.
 29. The systemaccording to claim 25, further comprising: a blocking unit for blockingsaid detected malicious packet.
 30. The system according to claim 29,wherein said blocking unit comprises a firewall which is instructed toreject said detected malicious packets.
 31. A system for detectingmalicious packets, the system comprising: a sequence number-comparingunit for comparing a sequence number of an input packet with sequencenumbers of one or more previously input packets; an acknowledgmentnumber-comparing unit for comparing an acknowledgment number of saidinput packet with acknowledgment numbers of said one or more previouslyinput packets; and a detecting unit for detecting a malicious packetwhen either said sequence number of said input packet matches at leastone of said sequence numbers of said previously input packets and/orsaid acknowledgement number of said input packet matches at least one ofsaid acknowledgement numbers of said previously input packets.
 32. Thesystem according to claim 31, wherein a database of sequence numbers ofpreviously input packets is used in comparing said sequence number ofsaid input packet with said sequence numbers of said one or morepreviously input packets; a database of acknowledgment numbers ofpreviously input packets is used in comparing said acknowledgment numberof said input packet with said acknowledgment numbers of said one ormore previously input packets, said system further comprising: adetermining unit for determining whether there is data in said inputpacket; a sequence number-adding unit for adding said sequence number ofsaid input packet to said database of sequence numbers when it isdetermined that there is data in said input packet; and anacknowledgement number-adding unit for adding said acknowledgment numberof said input packet to said database or acknowledgment numbers when itis determined that there is data in said input packet.
 33. The systemaccording to claim 32, wherein said database of sequence numbers is adatabase of a fixed size such that when said database of sequencenumbers is filled to capacity, adding said sequence number of said inputpacket to said database of sequence numbers additionally comprisesremoving an oldest sequence number from said database of sequencenumbers prior to adding said sequence number of said input packet tosaid database of sequence numbers; and said database of acknowledgmentnumbers is a database of a fixed size such that when said database ofacknowledgment numbers is filled to capacity, adding said acknowledgmentnumber of said input packet to said database of acknowledgment numbersadditionally comprises removing an oldest acknowledgment number fromsaid database of acknowledgement numbers prior to adding saidacknowledgment number of said input packet to said database ofacknowledgment numbers.
 34. The system according to claim 32, whereinsaid sequence number comparing unit uses a hash table when comparingsaid sequence number of said input packet with said sequence numbers ofsaid one or more previously input packets; and wherein saidacknowledgement number comparing unit uses a hash table when comparingsaid acknowledgment number of said input packet with said acknowledgmentnumbers of said one or more previously input packets.
 35. The systemaccording to claim 31, further comprising: a blocking unit for blockingsaid detected malicious packet.
 36. The system according to claim 35,wherein said blocking unit comprises a firewall which is instructed toreject said detected malicious packets.
 37. A computer systemcomprising: a processor; and a computer recording medium includingcomputer executable code executable by the processor for detectingmalicious packets, the computer executable code comprising: code forcomparing a sequence number of an input packet with sequence numbers ofone or more previously input packets; and code for detecting a maliciouspacket when said sequence number of said input packet matches at leastone of said sequence numbers of said previously input packets.
 38. Thecomputer system according to claim 37, wherein a database of sequencenumbers of previously input packets is used in comparing said sequencenumber of said input packet with said sequence numbers of said one ormore previously input packets and said code for detecting maliciouspackets further comprises: code for determining whether there is data insaid input packet; and code for adding said sequence number of saidinput packet to said database of sequence numbers when it is determinedthat there is data in said input packet.
 39. The computer systemaccording to claim 38, wherein said database of sequence numbers is adatabase of a fixed size such that when said database of sequencenumbers is filled to capacity, said code for adding said sequence numberof said input packet to said database of sequence numbers additionallycomprises code for removing an oldest sequence number from said databaseof sequence numbers prior to adding said sequence number of said inputpacket to said database of sequence numbers.
 40. The computer systemaccording to claim 38, wherein said code for comparing uses a hash tablewhen comparing said sequence number of said input packet with saidsequence numbers of said one or more previously input packets.
 41. Thecomputer system according to claim 37, further comprising code forblocking said detected malicious packet.
 42. The computer systemaccording to claim 41, wherein said code for blocking said detectedmalicious packet comprises code for instructing a firewall to rejectsaid detected malicious packets.
 43. A computer system comprising: aprocessor; and a computer recording medium including computer executablecode executable by the processor for detecting malicious packets, thecomputer executable code comprising: code for comparing anacknowledgment number of an input packet with acknowledgment numbers ofone or more previously input packets; and code for detecting a maliciouspacket when said acknowledgment number of said input packet matches atleast one of said acknowledgment numbers of said previously inputpackets.
 44. The computer system according to claim 43, wherein adatabase of acknowledgment numbers of previously input packets is usedin comparing said acknowledgment number of said input packet with saidacknowledgment numbers of said one or more previously input packets andsaid code for detecting malicious packets further comprises: code fordetermining whether there is data in said input packet; and code foradding said acknowledgment number of said input packet to said databaseof acknowledgment numbers when it is determined that there is data insaid input packet.
 45. The computer system according to claim 44,wherein said database of acknowledgment numbers is a database of a fixedsize such that when said database of acknowledgment numbers is filled tocapacity, adding said acknowledgment number of said input packet to saiddatabase of acknowledgment numbers additionally comprises removing anoldest acknowledgment number from said database of acknowledgementnumbers prior to adding said acknowledgment number of said input packetto said database of acknowledgment numbers.
 46. The computer systemaccording to claim 44, wherein said code for comparing further comprisescode for using a hash table when comparing said acknowledgment number ofsaid input packet with said acknowledgment numbers of said one or morepreviously input packets.
 47. The computer system according to claim 43,further comprising code for blocking said detected malicious packet. 48.The computer system according to claim 47, wherein said code forblocking of said detected malicious packet comprises code forinstructing a firewall to reject said detected malicious packets.
 49. Acomputer system comprising: a processor; and a computer recording mediumincluding computer executable code executable by the processor fordetecting malicious packets, the computer executable code comprising:code for comparing a sequence number of an input packet with sequencenumbers of one or more previously input packets; code for comparing anacknowledgment number of said input packet with acknowledgment numbersof said one or more previously input packets; and code for detecting amalicious packet when said sequence number of said input packet matchesat least one of said sequence numbers of said previously input packetsand/or said acknowledgement number of said input packet matches at leastone of said acknowledgement numbers of said previously input packets.50. The computer system according to claim 49, wherein a database ofsequence numbers of previously input packets is used in comparing saidsequence number of said input packet with said sequence numbers of saidone or more previously input packets; a database of acknowledgmentnumbers of previously input packets is used in comparing saidacknowledgment number of said input packet with said acknowledgmentnumbers of said one or more previously input packets; and code fordetermining whether there is data in said input packet, wherein saidcode for detecting malicious packets additionally comprises: code foradding said sequence number of said input packet to said database ofsequence numbers when it is determined that there is data in said inputpacket; and code for adding said acknowledgment number of said inputpacket to said database or acknowledgment numbers when it is determinedthat there is data in said input packet.
 51. The computer systemaccording to claim 50, wherein said database of sequence numbers is adatabase of a fixed size such that when said database of sequencenumbers is filled to capacity, adding said sequence number of said inputpacket to said database of sequence numbers additionally comprisesremoving an oldest sequence number from said database of sequencenumbers prior to adding said sequence number of said input packet tosaid database of sequence numbers; and said database of acknowledgmentnumbers is a database of a fixed size such that when said database ofacknowledgment numbers is filled to capacity, adding said acknowledgmentnumber of said input packet to said database of acknowledgment numbersadditionally comprises removing an oldest acknowledgment number fromsaid database of acknowledgement numbers prior to adding saidacknowledgment number of said input packet to said database ofacknowledgment numbers.
 52. The computer system according to claim 50,further comprising: code for using a hash table when comparing saidsequence number of said input packet with said sequence numbers of saidone or more previously input packets; and code for using a hash tablewhen comparing said acknowledgment number of said input packet with saidacknowledgment numbers of said one or more previously input packets. 53.The computer system according to claim 49, further comprising: code forblocking said detected malicious packet.
 54. The computer systemaccording to claim 53, wherein said code for blocking said detectedmalicious packet comprises: code for instructing a firewall to rejectsaid detected malicious packets.
 55. A computer recording mediumincluding computer executable code for detecting malicious packets, thecomputer executable code comprising: code for comparing a sequencenumber of an input packet with sequence numbers of one or morepreviously input packets; and code for detecting a malicious packet whensaid sequence number of said input packet matches at least one of saidsequence numbers of said one or more previously input packets.
 56. Acomputer recording medium including computer executable code fordetecting malicious packets, the computer executable code comprising:code for comparing an acknowledgment number of an input packet withacknowledgment numbers of one or more previously input packets; and codefor detecting a malicious packet when said acknowledgment number of saidinput packet matches at least one of said acknowledgment numbers of saidpreviously input packets.
 57. A computer recording medium includingcomputer executable code for detecting malicious packets, the computerexecutable code comprising: code for comparing a sequence number of aninput packet with sequence numbers of one or more previously inputpackets; code for comparing an acknowledgment number of said inputpacket with acknowledgment numbers of said one or more previously inputpackets; and code for detecting a malicious packet when said sequencenumber of said input packet matches at least one of said sequencenumbers of said previously input packets and/or said acknowledgementnumber of said input packet matches at least one of said acknowledgementnumbers of said previously input packets.